1. Field of the Invention
The present invention relates to phase shift keying communication systems.
2. Description of the Prior Art
In order to avoid inter-channel interference in high density, multichannel radio communication systems, strict control of the transmission output frequency spectrum in such communication systems is desirable. In addition, efficiency requirements in many communication systems, particularly satellite communication systems, dictate the use of non-linear amplifiers, such as Class B or Class C amplifiers. Thus, compatibility with non-linear amplifiers has become an important design criterion for transmitters in such communication systems.
To control such transmission frequency spectra, and eliminate spectrum splatter, conventional communication systems have utilized techniques known in the art as offset-keyed quaternary phase shift keying (OK-QPSK) and minimum shift keying (MSK). The implementation of such techniques, however, is relatively complex, particularly with respect to demodulation of the signal. Another technique to reduce the output frequency spectrum, a technique termed "unidirectional phase shifting" wherein the phase of a carrier signal is continuously advanced in one direction by a discrete step every time a data level transition occurs, is described in U.S. Pat. No. 3,659,202 issued Apr. 25, 1972 to H. Kaneko. In the system described in the Kaneko patent, however, each shift of carrier signal phase by the predetermined discrete step, for example .pi./4 radians, is indicative of a data transition, thus requiring the demodulator to be capable of demodulating phase increments equal to the discrete step (.pi./4). Such a demodulator is relatively complex and expensive. A communication system, having an output frequency spectrum comparable to that of OK-QPSK and MSK systems, that may be implemented in a relatively simple and inexpensive manner is thus desirable.